High Helium Reservoirs in the Four Corners Area of the Colorado Plateau, USA

Abstract:

Helium has unique thermodynamic properties required for the medical imaging industry, aerospace and other fields of high-tech manufacturing, and currently is in high demand. Helium is naturally produced in the Earth’s crust due to radiogenic decay of U and Th, however, the mechanisms of helium migration and retention in sedimentary basins are poorly understood. Improvements in the understanding of the helium system model are critical for exploration.

Oil and gas fields with economic helium (>0.3%) concentrations have been discovered in Paleozoic intervals in the Colorado Plateau, southwestern USA. Here we report new noble gas isotope and abundance data for gas samples (n=31), from actively producing Paleozoic formations within five fields: Ratherford, Tocito Dome, Pinta Dome, Navajo Springs, and Dineh-Bi-Keyah. Helium concentrations range from 0.01% to 7.9% with varying amounts of liquid and gaseous hydrocarbons, N₂, and CO₂.

Hydrocarbons and other crustal gases acquire their atmospheric noble gas composition by interaction with water. We present multi-stage gas, water and oil equilibration models to account for the observed atmospheric noble gas signatures and calculate gas/water and oil/water/gas ratios. Oil-dominated systems are explained by a closed system oil/water equilibration and subsequent admixture of air. Dry gas samples exhibit unusually high ²⁰Ne/³⁶Ar ratios (0.52 – 3.45) that are explained by partial exsolution followed by open system dissolution. This process removes the bulk of the N₂ gas phase (~90-99%) by dissolving it back into water. Observed gas-water interactions indicate extensive groundwater involvement in the concentration of economic ⁴He.

We show the amount of water required to have contacted the helium in place in the reservoir to be significantly larger than the current reservoir volume. Also, we find that the reservoir helium concentration cannot be sourced by in-reservoir decay of U and Th. Additional helium is therefore sourced from the Precambrian granitic basement and/or lateral gas migration over long distances consistent with the degree of water contact. Deep crustal lineaments in the Precambrian basement are in close proximity to the high helium fields, indicating that these structures are potentially serving as primary migration conduits via advective fluid flow.